Bio-inspired micropatterned thermochromic hydrogel for concurrent smart solar transmission and rapid visible-light stealth at all-working temperatures

Thermochromic hydrogels exhibit a smart capacity for regulating solar spectrum transmission, enabling automatically change their transmissivity in response to the ambient temperature change. This has great importance for energy conservation purposes. Military and civilian emergency thermochromic applications require rapid visible-light stealth (VLS); however, concurrent smart solar transmission and rapid VLS is yet to be realized. Inspired by squid-skin, we propose a micropatterned thermochromic hydrogel (MTH) to realize the concurrent control of smart solar transmittance and rapid VLS at all-working temperatures. The MTH possesses two optical regulation mechanisms: optical property regulation and optical scattering, controlled by temperature and pressure, respectively. The introduced surface micropattern strategy can arbitrarily switch between normal and diffuse transmission, and the VLS response time is within 1 s compared with previous ~180 s. The MTH also has a high solar-transmission regulation range of 61%. Further, the MTH preparation method is scalable and cost-effective. This novel regulation mechanism opens a new pathway towards applications with multifunctional optical requirements.


Supplementary Text
Note S1 The direction and application point of the applied pressure.
As illustrated in Fig. S1, the direction and application point of applied pressure are displayed.
The direction of the applied pressure is perpendicular to the surface of the MTH, and the application point is on two ends of the MTH.The pressure is exerted through mechanical device.

Note S2 Light scattering behavior of rough surfaces with Gaussian statics
In order to model and simulate the light scattering behavior, rough surfaces with Gaussian statics can be generated using a Monte Carlo method outlined by Garcia and Stoll [50].The principle of the Monte Carlo method is that the white noise random number is first transformed by fast Fourier transform, then the Gaussian rough surface power spectrum density is used to filter it in the frequency domain, and finally the inverse fast Fourier transform is used to obtain the 3D height data of the rough surface.
The specific steps of generating 3D random rough surface are as follows: (1) Select the horizontal and vertical lengths of desired random rough surfaces as Lx and Ly.
This region should contain enough peaks and troughs, and the sampling interval lengths (Δx and Δy) should satisfy the sampling theorem to ensure the accuracy of generating surfaces.Select the power spectral density function as W(K).Where Lx=Ly=5000 μm, Δx=Δy=20 μm.
(3) Generation of complex random numbers as follows： ( , ) exp( ) 44 The geometric characteristics of random rough surface can be described by root mean square height (Sq), root mean square slope (Sdq) and correlation length (l).
The Sq can describe the average undulation height of random rough surface, which can be expressed as: The Sdq can describe the average slope of random rough surface, which can be expressed as: The l can describe the density of random fluctuations of height.
The spatial distribution of surface roughness is usually interrelated, and the surface autocorrelation function (F(R)) can describe this correlation property, which can be expressed as The distance when the normalized correlation coefficient(ρ) is reduced to 1/e is defined as the correlation length.The normalized correlation coefficient(ρ) can be expressed as: When the Sq and incident zenith angle (θi) of rough surface and wavelength (λ) satisfies the relation as S-1, the geometric optical approximation can ensure the simulation accuracy of light scattering characteristics [51].
q cos / 0.17 Light scattering characteristics can be calculated using the geometric optics module in COMSOL Multiphysics @ software [52].This module uses ray tracing method to model the propagation of electromagnetic waves, where the propagation of waves is treated as rays and can be reflected, refracted or absorbed.The refractive index of the hydrogel varies between 1.26-1.35 with the change in wavelength (Fig. S16) [17].

Note S3 Bidirectional transmitted distribution function
The spatial distribution of transmitted light scattering on material surface is characterized by bidirectional transmission distribution function (BTDF) [53].Bidirectional transmission distribution function is defined as the ratio of transmitted radiation intensity and incident irradiance at a given zenith angle and azimuth angle, which can be expressed as: where ,

Note. S4 The detail about measuring force.
The pressure modules device has two force sensors, which is placed on the top of two ends of the MTH.The force is exerted on two force sensors, applying force on the MTH through the force sensors.For each sensor, the area contacting the MTH is 3.14 cm 2 .Therefore, the total value of applied force is the sum of the forces detected by each sensor.In the practice on windows in houses, the commercial electric linear actuator and mechanical device could be used to induce pressure.

Note. S5 Energy consumption analysis
To further investigate the energy-saving potential of this smart window, Energy Plus is used to establish a single-story house model and the "ideal-loads-air-systems" served with district cooling and heating sources is used to simulate the annual energy consumption.The results represent the direct effects of different window types (MTH, low-e, common glass) on the heating and cooling load of the building.Specifically, the building dimensions are 8 m (length) × 8 m (width) × 3 m (height), the details of the building model are displayed in Table S1, and the insulation material meets the requirements of ASHRAE Standard 90.1.In this model, the total glass area of the building is 32 m 2 , and the window wall ratio is 33.33%, evenly distributed on all four walls.The optical parameters of low-e and common glass used in the simulation were obtained from Ref.
[38], and the optical parameters of MTH were obtained from experimental measurements, as shown in Table S2.
It is worth noting that the thermochromic performance of micropatterned hydrogels based smart window is achieved through the Energy Plus built-in thermochromic function.In the simulation, the interior temperatures are designed 18 ℃ in winter and 26 ℃ in summer.In winter, if the temperature is lower than 18 ℃, the building will be heated.In summer, if the temperature is higher than 26 ℃, the building will be cooled.During the operation of EnergyPlus, we set the MTH parameters under different temperature conditions, at the beginning of a specific time step calculation, the MTH glass temperature of the previous time step is used to find the closest temperature condition, and the corresponding MTH parameter will be used for the current time step calculation.We set the time step of EnergyPlus calculation as 10 min to reduce the deviation of calculation results caused by the lag of this iterative method.We selected cities in different regions of the world to predict the energy-saving performance (Fig. 4E): Los Angeles, CA, U.S.; Miami, FL, U.S.; Brasilia, Brazil; Buenos Aires, Argentina; Accra, Ghana; Rome, Italy; Cape Town, South Africa; Cairo, Egypt; Nairobi, Kenya; New Delhi, India; Singapore, Singapore; Hong Kong, China; Beijing, China; Canberra, Australia.The weather data adopted in this investigation are from the online resource provided by the U.S. Department of Energy (DOE) [54]

Note S6 Equivalent conductivity of the MTH window
The conductivity of the MTH window is set as 0.9 W• (m‧K) -1 , which is an equivalent thermal conductivity of the glass/hydrogel/glass sandwich.Commonly, the conductivity of hydrogel is about 0.4~0.6W• (m‧K) -1 [55], and the conductivity of building glass is about 0.9~1.3W• (m‧K) -1 [56].According to the equivalent thermal conductivity equation

d d d d h h h h
the equivalent conductivity of the MTH window is calculated as 0.9 W• (m‧K) -1 , where d1, d2 and d3 are the thickness of glass, hydrogel and glass, respectively; h1, h2, and h3 are the conductivity of glass, hydrogel and glass, respectively.

Note. S7 Additional mathematical description.
The annual energy consumption (AEC) of a house can be divided to two parts as follows: AEC HEC+CEC = (Eq.S14) where HEC is the heating energy consumption, and CEC is the cooling energy consumption.The relationship between absorption, transmittance, and reflection is as follows:        Movie S2 (separate file).The MTH is applied to plant protection.The plant leaves turned from green to yellow after 20 min of sunshine without the MTH, while plant leaves remain green after 20 min of sunshine with the MTH.

Movie S3 (separate file).
The MTH is applied to dual-level anti-counterfeiting.When the MTH is touched by hands within 2 minutes, the MTH becomes from transparency to opaque.

Movie S4 (separate file).
The MTH is applied to dual-level anti-counterfeiting. the MTH shows characters below after pressed by a glass cover.
with different random characteristics can be obtained by substituting different power spectral density functions:

ii
 represent the zenith angle and azimuth angle of incident light, , tt  represent the zenith angle and azimuth angle of transmitted light, , it LL represent the radiation intensity of incident and transmitted light, i E represent the incident irradiance, i  represent the solid angle of incident light.

Table S1
Information of the building model applied in energy simulation.TableS2Optical properties for different windows used in simulation.The MTH is applied to visible stealth of military tank.Under emergency conditions, the military tank model is invisible transiently after releasing the pressure stimulus.